Our hypothesis is that in Myc overexpressing transgenic mouse mammary epithelial cells there is a selective/apoptotic pressure for secondary events resulting in the inactivation of p53. In contrast, the Myc/TGFalpha bitransgenic mammary epithelial cells, due to survival factor expression (TGFalpha), cells are not subject to p53 dependent apoptosis and growth inhibition. Transgenic strains of mice have successfully been used as model systems to further our understanding of the genetic changes occurring in human breast cancer. In this grant we propose to examine the functional status of p53 in MMTV-myc (myc) transgenic mice and in MMTV-myc/MT-TGFalpha (myc/tgf/alpha) bitransgenic mice. In our laboratory we have established transgenic mice that contain both the myc and tgf/alpha transgenes. We have previously shown in these transgenic mice that the Myc and TGFalpha proteins synergise to induce synchronous mammary gland tumors with a mean latency period of 66 days, while the expression of a single transgene (either tgf/alpha or myc) requires a long latency period and/or multiple pregnancies before tumors arise (1-3). Using cell-lines derived from the mammary tumors of these transgenic mice, we hope to gain a better understanding of the fundamental genetic alterations that lead to cellular transformation. Already, in preliminary data leading to this proposal, it has been observed that a cell line isolated from a Myc mammary tumor undergoes programmed cell death (apoptosis) in the absence of exogenous EGF while a Myc/TGFalpha cell line does not undergo apoptosis in the absence of exogenous EGF (4). To further our understanding of the mechanism of induction of apoptosis by Myc and survival from apoptosis by TGFalpha we have established 10 Myc and 6 Myc/TGFalpha cell-lines from independent mammary tumors. Specifically, the goal of this proposal is to conduct an in vivo and in vitro analysis of the function of p53 in mouse mammary epithelial cells: (1.) in the context of myc expression and (2.) in the context of myc/tgf/alpha coexpression. To further our studies on the genetic mechanisms of breast cancer we have also crossed the MMTV-myc transgenic mouse with a p53 null mouse to examine tumor latency and morphology in the presence or absence of the p53 allele. These experiments should prove invaluable in forwarding our understanding of the p53 requirement in both myc-promoted apoptosis and myc-promoted mammary tumorigenesis.